Built-Form, Mass and Energy Urban fabric · PDF fileBuilt-Form, Mass and Energy Urban fabric...

PLEA2012 - 28th Conference, Opportunities, Limits & Needs Towards an environmentally responsible architecture Lima, Perú 7-9 November 2012

Built-Form, Mass and Energy Urban fabric performance

Michele Morganti1, Anna Pages-Ramon

2, Antonio Isalgue

2, Helena Coch

2, Carlo Cecere

1

1SOSlab, Faculty of Engineering, DICEA Department Sapienza University of Rome, Rome, Italy

2Arquitectura, Energia i Medi Ambient, School of Architecture, UPC, Barcelona, Spain

ABSTRACT: The link between urban form and building energy demand is a complex balance of morphological,

constructive, utilization and climatic factor. Especially in the European compact city, where existing areas prevail on

much more energy-efficient new settlements, it is evident that operative ways to transform efficiently the building stock

have to be found. This paper explores the existence of a relation between built mass and energy demand depending on

urban form. Focusing on the compact city of Mediterranean climate, tests on different case studies simulations are

carried out. Results presented and discussed, point out that mass has strong relevance on energy demand and plays

an important role in reducing energy consumptions. This paper is a preliminary report of an ongoing research study

about one possible way to comprehend “metabolic rate” scaling law - the relationship between power and mass of a

complex system in its process - concerning urban fabric. This knowledge-base could help verify the accordance with

this rule on urban scale and give hints to conscious and effective built environment transformations towards more

efficient conditions.

Keywords: Built-form, Energy demand modelling, Energy performance, Urban fabric, Building mass.

INTRODUCTION

In recent years rapid population growth in urban areas

has established city as the first-rate contemporary human

habitat, at the same time giving rise to some concerns

about its “unsustainable” condition.

Currently it is widely known that in Europe complex

activity referring to the built environment is responsible

for 75% of GHG (Green House Gases) emissions and for

69% of final energy consumption [1, 2]. Recent studies

agree that there is an inevitable need to reduce GHG and

to take advantage of the opportunity to obtain complete

self-sufficiency through renewable energy by the middle

of this century [3, 4]. This is an extremely complex

process simultaneously requiring improvement in energy

performance in built environments, in order to reduce

global energy demand.

Especially in European compact cities, new settlement

models are still in negligible proportion compared to

dimension of the ordinary city, in whose goal of the

Sustainable Design is to be achieved. Then, it is

important to have indications on the investment and

effects of upgrade the built stock. Moreover it is now

widely accepted that urban scale has a first rate

importance in the building design process and its

correlated energy performance.

Residential building stock is responsible for 65% of

final energy consumption in buildings [5]. Urban form,

due to the obvious connection with morphology and

building systems, both at the urban and building scale,

mostly affects energy performance [6]. Our aim is to

study urban fabric energy demand, beginning with

building aspects. Studying building behaviour through

simulations can represent one possible method in

improving energy performance. Estimation of the effects

of built-form on mass and energy demand is the main

focus of this paper. Here we explore the existence of a

relation or at least a trend, between built mass and energy

demand. The purpose should be correlated to mass-

energy relation of different urban fabrics.

BACKGROUND

Recently, researchers have considered the influence of

complex environmental interactions occurring in the

urban context. Currently research efforts are focusing on

methods and techniques for energy simulation in order to

understand and, at the same time, approach different

levels, from the building to urban ones [7, 8, 9, 10].

Analytical methods that allow both use at various scales

and suitability to describe typologically uniform urban

fabrics, are crucial in this manner.

It has been observed that scaling laws are useful in

describing the complex structure of urban systems: e.g.

supply networks, transport and especially energy

consumption [11, 12, 13]. It is understood that modern

cities have a metabolic rate (mass-power ratio) that

approximately follows the living organism scaling laws

[13]. Nevertheless, it has not been verified that this

connection remains the same while studying the

phenomena at the urban and building scale and what kind


Figure 1: Aerial images of case studies.

of relationship exists between mass and power, or energy

depending on typologies and urban form.

Regarding the building scale, recent studies have

ascertained the existence of an interaction between built-

form typology and energy consumption, suggesting a

classification based on chronological, dimensional and

morphological factors [14].

The eventual interaction between the mass of different

built-form typologies and the energy consumption has

not yet been explored. Once established that mass is a

parameter firmly connected to both built-form and

“metabolism”, it could turn out to be the connection

between typology and energy performance. This aspect is

even more relevant in the context of European compact

city where we can easily find urban fabrics consisting of

fundamentally uniform morphological and typological

elements. The latter elements, as well as the other two

components of urban space - road networks and land

plots - are the most influential factors of energy

performance within an urban system.

BUILT-FORM, MASS AND ENERGY AS A

PERFORMANCE EVALUATION TOOL

Apart from testing the existence of a relation or a trend

between building mass and energy demand, depending

on built-form, our aim is to establish some key elements

of a knowledge-base for future analysis on conformity

with the metabolic rate scaling law at the urban scale.

Firstly, some typical urban fabrics were chosen,

consisting of “conventional” typology as a basic

component. Secondly, by using corresponding models,

mass and energy performance were evaluated in order to

ascertain suitable parameters that clearly express a

connection. This study is an initial approach for using

parameters (representative of built-form) as energy

performance evaluation tools on a homogeneous urban

texture.

CASE STUDIES

This study compares five residential typologies, different

in construction period, morphology and construction

system. Focusing on the metropolitan area of Barcelona,

tests through different case study simulations were

carried out, to represent conventional dwelling models

making up different and widespread urban fabrics built

during the historical reference period (Fig. 1, Fig. 2,

Table 1).

A - Historic Row House (1900) - Two level dwelling,

with narrow façades of 5 m (length/width ratio L/W ≈

0.4). The structure consists of load-bearing walls (15÷40

cm) above-ground masonry continuous foundation; dry

stone drain; floors with wooden beams and brick vaulted

ceiling; Catalan ventilated roof without thermal

insulation.


account parameters concerning building mass and their

energy demand. All other variables, except for those

relating to mass and energy of different typologies, have

been excluded to modelling. Consequently, to verify real

influence of built-form on energy performance, we

started considering only heating/cooling demands that

were most directly attributable to built-form. The study

does not have fully diagnostic aim, i.e. to provide exact

energy demands and mass, but rather to study the value

of this connection. The boundary clearly defined is the

building envelope. It corresponds, in some cases, to the

building studied, otherwise in other cases to the island or

to part of it.

The buildings analysed are the main components of

various existing urban fabrics in the European compact

city. The climate of reference is the Mediterranean

environment of Barcelona. In order to prevent formal and

building system singularities and spatial inconsistencies

of selected buildings from affecting interpretation of the

results, the cases underwent a process of filtering.

Conventional and coherent solutions were applied, as

substitution of the original solutions, if necessary.

Mass evaluation is based on calculation of effective

mass of the built elements without considering associated

mass due to the construction process, which is not part of

the building. The mass of supply networks,

urbanizations, exterior spaces and movable elements that

could be considered building elements (e.g. furniture,

electrical household appliance, etc.), were not included in

the calculation. All building systems were taken into

account using default weight values [17]. The mass

assessment process started from finding volume and

density properties of different construction materials.

Facing complex recent components we used the IteC

Database, which disassembles construction elements

regarding weight and material [18]. All components were

grouped into building subsystems (e. g., foundation,

structure, envelope, etc.), while simultaneously

estimating the impacts of singular subsystems on the

overall value. Results shown are expressed in metric

Tons referring to thermal conditioned areas as specific

weight (Tm/m2).

Energy demand was evaluated by modelling on Lider

(v. 1.0 July 2009), a program associated with the Spanish

Technical Building Code approved in March 2006 [19].

The derived demand values were separated into two

components: heating and cooling. Taking into account

eight possible orientations mean, minimum and

maximum values were obtained. Modelling of all

building subsystems, fixed shadows (balconies, walls,

etc.) as well as internal partitions were carried out

starting from detailed acquired data for each selected

case. Urban obstructions were taken into account by

modelling effective urban fabric geometrical properties.

Concerning user dependent factors, i.e. hygrometry,

ventilation rate and movable solar protections, default

values were attributed. The latter is taken into account by

simulation through two solar mean reduction factors

(summer and winter). Lider provides energy demand

measured in kWh/(m2year). The surface considered

refers to a thermally conditioned area.

RESULTS

The results presented and discussed are three parted.

First, the building mass evaluation process is presented

and analysed, then energy demand for heating and

cooling is also presented, and finally their relationship is

explained.

Table 2 shows the built mass referring to useful floor

area. First of all we can observe that recent buildings (B -

E) are heavier (expressed as specific weight) than

historical buildings constructed before 1960 (A - C - D).

Moreover, the apartment blocks (C - D - E) are slightly

lighter per unit area than row houses (A - B). Therefore,

the heavier building is case B (recent row house) while

the lighter building is case C (historic apartment block).

Table 2: Built mass.

Case study Mass (Tm/m2)

A 1.53

B 2.58

C 1.11

D 1.24

E 1.65

The mass of the modern cases (Tm/m2 of thermal

conditioned areas) is greater mainly because of mass

properties in construction systems based on concrete

flooring and also because of more unconditioned spaces

in the buildings - especially underground car park-.

Table 3 shows results regarding annual energy demand

referring to heating and cooling of different urban

fabrics. Also in this parametric representation it is

possible to observe a clear distinction between historic

buildings (A - C - D) and recent buildings (B - E). The

former have envelopes without thermal insulation, while

the latter are built according to thermal regulations that

restrict heat transmission coefficients (U). As further

proof, case B and E have a conditioning energy demand

of about 40 kWh/m2y, roughly half that of the other

cases (Table 3).

Cooling energy demands of the historic urban fabric

is low, 15% of heating demand, while in the case of

contemporary urban fabric; cooling demand represents

over 50% of heating demand. This is mainly due both to

the presence/absence of thermal insulation, and to urban

form properties, e.g. density, geometry, orientation. The

first issue - along with modern envelopes which have


lower thermal inertia -, produces higher cooling

demands, while the second affects solar radiation access.

Results due to possible different orientations illustrate

that variation is broader in modern building than in

historic buildings. Historic buildings demonstrate similar

energy performance, while contemporary buildings are

more sensitive to orientation shifts. For example, case E

shows great variation: the worst orientation has an

energy demand 84% greater than the best orientation.

Table 3: Heating and cooling energy demand.

Heating (kWh/

m2year)

Cooling (kWh/

m2year)

Heating and cooling (kWh/m2year)

Average Min. Max.

A 86.88 3.22 90.10 86.57 93.12

B 21.85 13.32 41.04 33.17 47.01

C 79.33 8.17 87.50 85.48 89.28

D 77.52 5.74 83.26 78.29 89.28

E 22.34 11.75 39.64 25.74 47.23

Table 4 and Figure 3 show the existing tendency

between mass properties and building energy demand.

Referring to thermally conditioned areas, results point

out that mass has strong relevance on energy, as a rough

approximation described by the fitting as:

y = 98.952 x-1.998

where y represents the energy demand (kWh/Tm year)

and x the built mass (Tm/m2 of thermal conditioned

area).

Table 4: Built mass and energy demand.

Case study Mass

(Tm/m2)

Energy demand

(kWh/Tm year)

A 1.53 58.9

B 2.58 15.9

C 1.11 78.9

D 1.24 67.0

E 1.65 24.1

Hence the greater the mass per conditioned square meter

an urban fabric has, the less energy demand it

demonstrates for heating and cooling per mass unit.

Furthermore, the expression exponent near -2 suggests

that mass plays an important role in reducing energy

consumption (if it were -1 means that energy per unit

area was constant). The reason for this should be due to

the relationship between mass and energy for modern

and historic urban fabrics. Modern urban fabrics have

much heavier built-form building systems (mass per

conditioned unit area) and at the same time (because of

thermal regulation) they have a lower energy demand.

Figure 3: Relation between built mass and energy demand.

Dots, computed and experimental values; continuous line,

least-squares fit.

CONCLUSIONS

This study provides a preliminary knowledge-base on

finding a relationship between mass and energy

consumption of different urban fabrics.

The analysis carried out on case studies prove that

there is a relation between mass and energy demand (for

heating and cooling) in the Mediterranean climate urban

fabrics, which adopt built-form typologies and

constructive systems widely spread in the Barcelona

metropolitan area.

Nevertheless future studies that increase the number

of case studies (without considering it part of the

identification of the relationship) and expand the research

field to include mass and energy, are required. Moreover

for a complete comprehension of metabolism of urban

fabric, more built-forms and behaviour climates should

be analysed.

Other aspects are also to be considered, e.g. transport,

lighting, hot water, electrical appliances, etc., which

could lead to verifying accordance with this rule on the

urban scale and give hints to conscious and effective

built environment transformations, moving towards more

efficient conditions.

ACKNOWLEDGMENTS

This work has been supported by MICINN project

ENE2009-11540. M. M. acknowledges Sapienza

University of Rome for PhD fellowship.

y = 98.952x-1.998

R² = 0.8533

0

10

20

30

40

50

60

70

80

90

0 1 2 3 Tm/m2

kWh/

Tm year


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